Apparatus for corrosion prevention



2,847,375 APPARATUS FOR CORROSIN PRVENTION f Gordonl I. W. Murphy,Houston, Tex.,1assignor to The Texasv Company, New York, N. Y., acorporation of Delaware y Application July 13, 19753, 'Serial No.367,678

2 claims. (cnam-197) This invention relates to a ,novel method of andapparatus for protecting against. corrosionua metal member having a partthat is normally wet only intermittently byV electrolyte from a bodyofelectrolyte. More' par,- ticularly the invention concerns protectingagainst corro- -sion such structural members as steel piling andoil wellp ipes which are located in the `water of seas, lakes, and rivers.

In the drawingsy illustrating the present invention:

Figure l is a graph showing the relative amounts of corrosion occurringon various 'parts of an unprotected steelmember exposed to the sepa,based on Cor-rosiou, vol. 5, page 292; and

Figures 2 and 3 arek vertical sectional viewsjparts being in elevation,showing two forms -of apparausl in accordance with the invention.

When metal structural members are employed in or `near a body of water,for example when offshore oil Well drilling platforms are supported bysteel piles driven into the mud at the bottom of the sea, seriouscorrosion occurs not only of the part of the piling which iscontinuously submerged but also of the part above the low tide markwhich is only intermittently wet by changes in the tidal level and bythe splashing of waves.

Figure l shows thaty the maximum corrosion rate is in the splash zone(zone 2) above high tide, but that substantial corrosion also occurs inthe tidal zone (zone 3) between the low and high tide marks. Cathodicprotection reduces corrosion of submerged parts of such a steel member(zone 4; and zone 3 only periodically), but serious corrosion still hasoccurred above the low tide mark. Prior to the present invention no waywas known for cathodcally protecting parts of a steel pile or otherstructural metal member against corrosion when not submerged within thewater.

In the following description the term sea water will be used forsimplicity to describe one type of electrolyte to which metal membersare exposed, but it is to be understood that the principles of theinvention apply also when metal members are subject to corrosion byother electrolytes.

In accordance with the present invention there is provided a novelmethod for cathodically protecting against corrosion a metal memberhaving at least a part that is normally wet only intermittently by seawater. In its broadest aspect my invention involves artificiallymaintaining a body of an electrolyte continuously in contact with theexposed parts of the metal member while impressing a protectiveelectrical current on the member to counteract its normally anodic state.and make it cathodic.

In the preferred embodiment of the invention an electrolyte ismaintained in contact with the exposed parts of the metal member bysurrounding such parts with a bibulous mass of material soaked withelectrolyte. Usually the soaking electrolyte is the sea water, to whichthe bibulous mass is exposed, Sea water is absorbed by the bibulous massand retained therein during periods when in the sea nearby or in thebibulous mass; or bytheV use 2,847,375 Patented Aug. 12, 1958 ice themetal parts are out of Contact with the body of seav water. The presenceof this retained electrolyte around the highly corrodable parts of themetal structural member makes it possible to impress a protectiveelectrical current continuously thereon.

A protective electrical current can be impressed -on the structuralmember by electrical connection tothe negative pole of a battery orother source of direct current whose positive pole is connected to anearby anode of metal, graphite, or the like, which is submerged eitherof a sacricial anode of a. metal or .alloy jwhic'h has a higher anodicsolution potential than does the material of the structural member. Forsimplicity Izprefer `to use a sacricial metal anode which, when thestructural member is iron, .advantageously is formed of such metals .as

zinc, aluminum, magnesium, or alloys of these metals One specificapparatus for performing the'method clel f: scribed above is shown inFigure 2,' wherein a steel structural member 11 such as a vertical. pileor oil wellv pipe extends from above the surface of the sea 13` down.into the latter and into the mud 15 at the bottom. Thev part of themember 11 extending from slightly below the low tide mark 17 to slightlyabove the top of the splash zone 19 is surrounded by a porous shell 21spaced annularly from member 11 and having a bottom 23, to form a hollowcontainer. Shell 21 can be formed of any suitable sea-water-permeablematerial such as heavy cloth, woven wire screen, wood slats, or porousplastic.

Shell 21 is packed with a bibulous mass 25 of material which will absorbwater from the sea 13 to keep the wall of member 11 permanently wet fromslightly above the top of splash zone 19 down to low tide mark 17.Suitable materials for the bibulous mass 25 are bonded or unbondedsawdust, or such mineral particles as bentonite clay, gypsum, or amixture of gypsum with bentonite (20%) and sodium sulfate (5%), but itis evident that many other bonded or unbonded absorptive materials couldbe used within the scope of the invention. Extension of the shell Z1 andthe mass 25 to a position below the low tide mark assures that the mass2S will remain wet by its wick action on the water from the sea 13. Thiswick action is supplemented by the water absorbed at high tide and bythe splashing of waves.

A protective electrical current advantageously is impressed on thestructural member 11 by embedding a sacrificial anode 26 of magnesium orother sacrificial anodic material within the bibulous mass 25 in spacedrelationship to member 11 and the walls of shell 21, and connecting theanode to the member 11 by an electrical connection 27.

With the construction described above the whole structural member 11 isconstantly protected against corrosion even when parts of it are exposedabove the low tide mark or are splashed by waves.

In the modication shown in Figure 3 the structural member 11 issurrounded by a bibulous mass 25 contained within a shell 21', as inFigure 2. Here, however, the sacrificial anode 26' is not embeddedwithin the mass 25 but is suspended within the sea 13' below the lowtide mark 17', and is electrically connected to the mass 11 by anexternal electrical connection 27. It is essential in this modificationthat the bibulous mass 25' be in electrical contact with the sea 13 atall times if continuous cathodic protection is to be obtained.

From the foregoing description it is evident that there have beendevised an improved method and apparatus for protecting metal structuralmembers from corrosion when they are located at places where parts areonly intermittentlywet. While the principles have been describedprimarily as applied to such structural members asvertical steel pilesand offshore oil Wellpipes, it is apparent that they apply equally tothe protection of other types of structural members subjected to similarcorrosive conditions such as beams, plates, and more complex structures,whether vertical or otherwise.

Obviously, many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore only such limitations should be imposedas are indicated in the appended claims.

I'claim:

1. In combination a metal member which extends from above the surface ofa body of an aqueous electrolyte down into said body below the lowestsurface level reached thereby, said member having at least a partthereof above said lowest surface llevel that is normally wet onlyintermittently by aqueous electrolyte from said body; a bibulous mass ofmaterial coextensively surrounding and contacting said part andextending down below said lowest surface level; an aqueous electrolytepermeable shell coextensively surrounding and contacting said mass andretaining said mass in contact with said metal member; a sacrificialmetal anode of metal which has a higher anodic solution potential thandoes the material of said metal member, said anode being entirelyimbedded within said mass in spaced relationship to said member and saidshell and connected electrically to said metal member in a manner topermit autogenous generation and impression of electrical current uponsaid metal member.

2. In combination a structural iron member which extends from above thesurface of a body of water down into said body below the lowest surfacelevel reached thereby, said member having at leasty a part thereof abovesaid lowest surface level that is normally wet only intermittently byAwater from said body, a bibulous mass of material coextensivelysurrounding and contacting said part and extending above and below saidlowest surface level, a shell of water permeablematerial coextensivelysurrounding and contacting said mass in a manner to retain said mass incontact with said member, the top edge of said shell being annularlyspaced from said member, the bottom edge of said shell being in directcontinuous contact with said member; a sacricial anodeof metal which hasa higher anodic potential than iron, said yanode being entirely imbeddedwithin said mass in spaced relationship to said member and said shell,said anode ,exf tending from above to below said lowest surface leveland being' connected electrically to said iron member to permitautogenous generation and impression of electrical current upon saidmember.

Y References Cited in the le of this patent y UNITED STATES PATENTSScott Feb. 24, 1942

1. IN COMBINATION A METAL MEMBER WHICH EXTENDS FROM ABOVE THE SURFACE OFA BODY OF AN AQUEOUS ELECTROLYTE DOWN INTO SAID BODY BELOW THE LOWESTSURFACE LEVEL REACHED THEREBY, SAID MEMBER HAVING AT LEAST A PARTTHEREOF ABOVE SAID LOWEST SURFACE LEVEL THAT IS NORMALLY WET ONLYINTERMITTENTLY BY AQUEOUS ELECTROLYTE FROM SAID BODY; A BIBULOUS MASS OFMATERIAL COEXTENSIVELY SURROUNDING AND CONTACTING SAID PART ANDEXTENDING DOWN BELOW SAID LOWEST SURFACE LEVEL; AN AQUEOUS ELECTROLYTEPERMEABLE SHELL COEXTENSIVELY SURROUNDING AND CONTACTING SAID MASS ANDRETAINING SAID MASS IN CONTACT WITH SAID METAL MEMBER; A SACRIFICIALMETAL ANODE OF METAL WHICH HAS A HIGHER ANODIC SOLUTION POTENTIAL THANDOES